The present invention concerns a brake pressure limiting device for a dual-circuit vehicle brake system. More particularly, the invention concerns a device of this type which incorporates two equi-axially arranged valve bodies each having one inlet adapted for connection with a master brake cylinder and at least one outlet adapted for connection with a vehicle brake line. An axially displaceable valve element is disposed within each valve body and arranged to move between an open position, in which the pressure at the valve body inlet is transmitted to the outlet, and a closed position isolating the pressures at the inlet and outlet. The pressure limitation effected by closing of the two valve elements is controlled by a common actuating member as a function of the vehicle load.
In a known brake pressure limiting device which is disclosed in the West German Offenlegungschrift No. 1,915,485 one valve body is inserted inside another and the two are closely connected with each other. A valve element is supported within each of these valve bodies and arranged to move linearly along a common longitudinal axis. A load-dependent actuating member is arranged to apply a force at the free end of one of the valve bodies. This force acts by way of a pressure member on the valve element within this valve body ("first valve element"), and presses it back in the direction of opening. As this valve element is pushed back by the pressure element, it in turn presses back on a shaft of the valve element in the adjacent valve body ("second valve element") so that this second element is also opened. When the two valves are open, the pressures at the inlets in the two valve bodies, which are connected with the master brake cylinder, determine the pressures at the associated valve body outlets that are connected, via brake lines, to the wheel brake cylinders
As is well known, the pressure delivered by the master brake cylinder is increased when the brake is actuated. When this pressure attains a value which, when multiplied by the effective surface area of the valve, corresponds to the force of the actuating member, the first valve element in the valve body pushes back on the pressure element, and thus on the actuating member, and closes again. The second valve element in the adjacent valve body is subject to two equal pressure forces in opposition. A special spring within this valve body takes care that this valve is then also closed.
The two valve elements are designed in such a manner that a small clearance remains between them when they are in the closed position.
When the pressure determined by the master brake cylinder drops, the actuating member again pushes back on the first valve element of the valve body so that this first element is thereby opened. As a result, the pressure connection between the master brake cylinder and the associated brake line is restored. The second valve element of the adjacent valve body is opened only somewhat later than the first since the play between the two valve elements and the spring force acting on the second valve element must first be overcome. Consequently, as may be seen, there exists by necessity a certain asymmetry in the response of the two valves.